Field of the Invention
The present disclosure relates to an image forming apparatus based on electrophotography, a method for controlling an image forming apparatus, and a non-transitory storage medium encoded with a computer readable program.
Description of the Related Art
In an image forming apparatus based on electrophotography, as means for removing residual toner such as un-transferred toner or post-transfer residual toner on a latent image carrier, a cleaning device based on a blade cleaning system is known for example that has a flat-shaped cleaning blade formed of an elastic body and brought into contact with a surface of the latent image carrier to thereby remove the residual toner on the latent image carrier.
In recent years, the electrophotographic image forming apparatus has been required to have toner particles with a reduced particle size for the sake of enhancement of the image quality. As a method for obtaining such toner particles, a polymerization method such as emulsion polymerization or suspension polymerization method for example is used.
However, as the particle size of toner particles is reduced, the adhesion between toner particles and the latent image carrier increases, which makes it difficult to remove the residual toner on the latent image carrier. Particularly in the case where toner produced by the polymerization method, namely so-called polymerized toner is used, the shape of toner particles is close to a sphere. The toner particles thus roll on the latent image carrier to pass through the cleaning blade, namely a cleaning failure called “pass-through” is likely to occur. A resultant problem is therefore the increased difficulty in removing residual toner on the latent image carrier.
Moreover, if some toner particles pass through the blade, a toner aggregate with the toner particle at the core is formed on the latent image carrier, to generate a granular blank (granular noise) in a solid-image printed portion.
In order to address the quality problems such as “pass-through” or “granular noise,” currently a lubricant is supplied onto the latent image carrier to reduce the adhesion between toner particles and the latent image carrier and, in this condition, cleaning is performed.
There are some methods for supplying the lubricant onto the latent image carrier, including a lubricant application system according to which a brush is brought into contact with a rod-shaped lubricant to scrape the lubricant and supply the lubricant to the surface of the latent image carrier, and an external-addition-to toner system according to which a toner containing a lubricating external additive is applied to form a toner image and the lubricant is thus supplied.
The external-addition-to-toner system is advantageous in terms of installation space and cost, since application devices such as lubricant rod and brush are unnecessary. However, regarding the external-addition-to-toner system, the lubricant is predominantly consumed when an image having a low area ratio is printed, resulting in decrease of the amount of the lubricant in a development device. The decrease of the amount of the lubricant in the development device causes the lubricant to be supplied unevenly to the surface of the latent image carrier, leaving a region where no lubricant is present. When the amount of the lubricant supplied per unit area decreases, the lubricant cannot be supplied enough to reduce the amount of the toner adhering to the latent image carrier to cause the above problems of the quality.
Regarding this, Japanese Laid-Open Patent Publication No. 2009-58732 discloses a system according to which a lubricant is applied onto an intermediate transfer medium, and a potential difference between the intermediate transfer medium and a photoreceptor causes the lubricant to be supplied onto the photoreceptor. At this time, the surface potential of the photoreceptor (transfer potential difference) is changed to control the amount of the lubricant supplied in the axial direction.
Japanese Laid-Open Patent Publication No. 2002-258707 discloses a cleaning system having a developer of the external-addition-to-toner type, a cleaning roller, and a blade. In accordance with the system, when a patch image is to be formed, the bias is changed so as not to collect the toner and the lubricant by the cleaning roller and the toner and the lubricant are caused to reach the blade.
Japanese Laid-Open Patent Publication No. 2000-132003 discloses a system according to which a zinc stearate (lubricant) supply mode is provided separately from an image forming mode and the development bias is changed in the supply mode.